1. Field of the Invention
The present invention relates to a backlight unit and a liquid crystal display apparatus. More specifically, the present invention relates to a backlight unit provided precisely underneath a display device such as a liquid crystal panel for illuminating the display device with light and a liquid crystal display apparatus comprising the backlight unit.
2. Description of the Related Art
Conventionally, in a liquid crystal panel, a pair of substrates is filled with a liquid crystal. A voltage is applied between a plurality of pixel electrodes arranged in a matrix on one of the pair of the substrates and an opposed electrode provided on the other substrate so as to oppose the pixel electrodes. Thus, the alignment of the liquid crystal between each of the pixel electrodes and the opposed electrode is changed, thereby changing a light transmittance of a pixel portion corresponding to the pixel electrode.
In the liquid crystal display apparatus, a backlight unit provided on the rear surface side of the liquid crystal panel illuminates a liquid crystal panel with light. Thus, an image is displayed through the liquid crystal panel which includes pixel portions having changeable light transmittances.
As the backlight unit, a backlight unit of the edge light system using a light guide plate is widely used. In the backlight unit of the edge light system, a light guide plate is provided on the rear surface side of a liquid crystal panel so as to face the liquid crystal panel. The liquid crystal panel is illuminated with light originated from a linear light source such as a fluorescent tube provided on a side surface (edge) side of the light guide plate and guided to an entire surface of the liquid crystal panel by the light guide plate.
FIG. 3 is a cross-sectional view of a conventional liquid crystal display apparatus showing the schematic configuration of the major parts thereof.
As shown in FIG. 3, a liquid crystal display apparatus 101 comprises a liquid crystal panel 102, and a backlight unit 103 which is provided on the rear surface side of the liquid crystal panel 102.
The backlight unit 103 includes a light source 104, reflectors 105, a light guide plate 106, a reflection sheet 107, a chassis 108 and an optical sheet 109.
The light source 104 comprises a pillar shape fluorescent lamp (or a light emitting diode) as a linear light source. The light source 104 is provided along side surfaces of the light guide plate 106.
Each of the reflectors 105 has a U-shaped cross-section with an opening facing a side surface of the light guide plate 106. The reflectors 105 are provided along the side surfaces of the light guide plate 106 such that they surround the light source 104. The light from the light source 104 is reflected by each of the reflectors 105 and guided toward a side surface of the light guide plate 106.
The light guide plate 106 is configured such that the light from the light source 104 and the reflectors 105 enters through a side surface, propagates inside, and is emitted from a front surface, which serves as an emitting portion, toward the liquid crystal panel 102. The light guide plate 106 serves as a planar light source.
The reflection sheet 107 is provided on the rear surface side of the light guide plate 106. The reflection sheet 107 reflects light emitted from the rear surface of the light guide plate 106 so that the light goes back through the light guide plate 106 toward the liquid crystal panel 102.
The chassis 108 has a concave shape, which houses the light source 104, the reflectors 105, the light guide plate 106, the reflection sheet 107 and the like.
The optical sheet 109 is provided on the front surface side of the light guide plate 106. Thus, the liquid crystal panel 102 can be uniformly irradiated with light emitted from the light guide plate 106 to the liquid crystal panel 102.
Conventionally, a backlight unit including two lamps as a light source, which are located such that light enters from all of the four side surfaces of the light guideplate 106, has been known (see, for example, Japanese Laid-Open Publication No. 4-329520). With such a structure, an amount of light which enters the light guide plate 106 from the light source in the liquid crystal display apparatus 101 can be increased.
FIG. 4 is a plan view of the structure of the backlight unit 103 shown in FIG. 3, as viewed from the emitting portion side.
The cross-sectional view of the liquid crystal display apparatus 101 shown in FIG. 3 corresponds to a cross section of the backlight unit 103 taken along line X′-Y′ of FIG. 4.
As shown in FIG. 4, the backlight unit 103 comprises two L-shape fluorescent lamps each extending over two of the side surfaces of the light guide plate 106 having a rectangular shape. The reflectors 105 are located so as to surround the fluorescent lamps.
In the structure described above, light enters from all of the four side surfaces of the light guide plate 106. Thus, compared to a structure in which straight-tube shape fluorescent lamps are located along two of the side surfaces of the light guide plate 106, an amount of light which enters the light guide plate 106 can be significantly increased.
In the conventional backlight unit 103, each of the two L-shape fluorescent lamps is located so as to extend over two of the side surfaces of the light guide plate 106 such that light enters from all of the four side surfaces of the light guide plate 106. Thus, the light guide plate 106 cannot be fixed to the chassis 108. Therefore, if a shock is given to the light guide plate 106 in a vertical or horizontal direction, the light guide plate 106 may directly contact a fluorescent lamp, i.e., the light source 104.
In order to prevent such a problem in the backlight unit 103, as shown in FIG. 4, the lengths of the L-shape fluorescent lamps are shortened to provide pillar shape light guide plate fixing members 110 for supporting the light guide plate 106 on two diagonal corners of the light guide plate 106. Thus, there is a problem that an amount of light entering the light guide plate 106 from the L-shape fluorescent lamps becomes smaller. The backlight unit 103 of a high luminance cannot be achieved.
If the fluorescent lamps are shortened in order to facilitate supporting the light guide plate 106, light does not enter from a portion of the side surfaces of the light guide plate 106 which the fluorescent lamps do not face. Thus, there is another problem that display is dark in corners of a display area of a screen of the liquid crystal panel 102.
There is still another problem that additional cost and processes are required to form the light guide plate fixing members 110.